BCS-CRM 531 Low Iron Sand , available in bottles of 100g of finely divided material for chemical analysis passing a nominal 250 micron aperture. It will remain stable provided that the bottle remains sealed and is stored in a dry atmosphere. When the bottle has been opened the lid should be secured immediately after use. In order to ensure that a fully representative sample is taken, users should take a minimum sub-sample size of 1.0 g. Users of this material should be aware that the use of a smaller sub-sample size will invalidate the certified values and the associated 95% confidence limits. Provided that the material is stored in a suitable environment there will be no contribution to the uncertainty from the long term stability of this CRM.